Anticoagulants are useful therapeutic agents in the pharmacological treatment of, for example, acute deep venous thrombosis, pulmonary embolism, acute arterial embolization of the extremities, myocardial infarction, and disseminated intravascular coagulation. Prophylactic administration of anticoagulants is believed to prevent a recurrance of embolism in patients with rheumatic or arteriosclerotic heart disease and to prevent certain thromboembolic complications of surgery. Administration of anticoagulants has also been indicated in the treatment of coronary artery and cerebrovascular disease. Arterial thrombosis, particularly in arteries supplying the heart muscle and brain, is a leading cause of death.
Blood coagulation depends on many enzymes and cofactors for its success and proceeds by two separate pathways, the intrinsic and extrinsic pathways. In the intrinsic or cascade pathway, all factors are present in circulating blood, but coagulation by this mechanism once initiated takes several minutes to accomplish. In the extrinsic pathway certain lipoproteins, i.e., Factor III, not normally present in circulating blood, are released by damaged cells, and coagulation begins within seconds. These pathways converge at a point in the coagulation process where Factor X.sub.a, together with Factor V and calcium ion, form the prothrombinase complex to catalyze the conversion of prothrombin to thrombin. Thus, Factor X.sub.a is of vital importance to both coagulation pathways and its inhibition would reduce blood coagulation regardless of cause. While several inhibitors of Factor X.sub.a are known, such inhibitors are either of the chloroketone class which are quite toxic or are non-specific inhibitors of serine proteases. Thus, a specific and non-toxic inhibitor of Factor Xa would have important therapeutic value.
Leeches have been used medicinally since antiquity. The medicinal use of leeches in the early 19th century, caused the near extinction of the species Hirudo medicinalis and caused Russia to impose quotas of its export. More recently, leech secretions have been more scientifically studied and have been found to contain a variety of biological products having a wide spectrum of biochemical and pharmacological activities such as anticoagulant, antimetastatic, anaesthetic, antibiotic, and vasodilator. For example, Hirudin isolated from the salivary gland of the leech Hirudo medicinalis is the most specific and potent thrombin inhibitor known. Further, Hementin which is isolated from the salivary gland of the leech Haementeria ghilianii is a fibrin(ogen)olytic enzyme of reported high molecular weight. It is reportedly the anticoagulant principle of this leech. This enzyme degrades fibrinogen and fibrin rather than activating the host fibrinolytic system or inhibiting the coagulation system.
Applicant has identified and isolated from the saliva and extracts of salivary gland tissues of the leech Haementeria ghilianii, a proteinaceous substance which is a specific inhibitor of Factor X.sub.a and which is a useful inhibitor of blood coagulation.